This invention relates to a multilayer card, in particular credit card, identity card, bank card or the like, having a security element with diffraction structures, in particular holographic relief structures, and to methods for producing the card.
It is commonly known to equip cards, in particular bank, credit or identification cards, with security elements in the form of holograms, kinegrams or the like. The security elements have very specific optical effects which are dependent on the viewing angle and can be checked visually without aids and whose production requires considerable technological effort. The presence of such a security element with the defined optical effects is evaluated as an authenticity criterion, i.e. the presence of such a security element is taken as an indication of the authenticity of the card.
Security elements having diffraction structures are usually glued on the outer surface of the cards. If this is done by the so-called hot stamping technique, as described in German laid-open print 33 08 831, these security elements cannot be removed from the card without destruction after being glued on the card. Transfer of an authentic security element from an e.g. expired card to a counterfeit new card can thus be excluded. However, it is disadvantageous that these elements, precisely because they have no inherent stability, are extremely thin and thus mechanically very sensitive. Such elements usually wear out relatively fast during use of the card, i.e. abrasion destroys the layer structure so that the specific optical effect resulting from the diffraction structures is lost. Although the security value of holograms, kinegrams, etc., for cards is rated relatively high, such security elements are thus not very suitable for cards with a given long duration, such as personal identification cards, passports, driver's licenses, etc.
There have been various attempts to embed security elements with diffraction structures in the inside of the card in order to avoid mechanical wear. However, it has turned out that customary holograms, kinegrams, etc., are either completely destroyed by the action of heat and pressure during the laminating process, or so greatly impaired in quality that further use is impossible. There are proposals to handle the card areas where the diffraction structures are embedded more gently during the laminating process than the areas having no diffraction structures. According to the proposals, these areas are exposed to less high temperatures and possibly also lower laminating pressure during production. However, such measures, as described e.g. in EP 0 013 557, could hardly reduce the degree of damage to the security elements. Instead, the less stressed card areas have a poorer film bond, which is visually recognizable and which tends to lead to splitting of the card layers in these areas as a result of bending stresses.